Process for maintaining or regulating the efficacy of cyanide melts



Patented Dec. 8, 1936 A omrEosTA Es PATENT OFFICE PROCESS FOR MIAINTAINING R REGULAT- ING THE EFFICACY OF CYANIDE MELTS Walter Beck, Frankfort-on-the-Main, and Klaus Bonath, Cronberg-in-Taunus, Germany .No Drawing. Application August 4, 1933, Serial 6 3 n Germany August 13, 1932 6 Claims.

- that the latter may be formed gradually by the decomposition of such cyanides in consequence of the air or the iron articles to be carburized acting on the melt at the high temperatures employed. Therefore, the efiiciency of the melt containing cyanides decreases, although the quantity of cyanide present would be sufiicient to ensure a perfect carburizing effects The alkaline substances, therefore, exert obviously a stabilizing action upon the cyanides since their cementing or carburizing activity depends again upon the rate at which they are decomposed.

The object of this invention is, a process by which the stabilizing action of alkaline substances formed by the decomposition of cyanides present .in the melt can be prevented and-the efficacy of the carburizing substances can be restored or regulated as the case may be.

We have found that this stabilizing action of alkaline substances, formed during use, or present in the melt can be prevented or counteracted by the addition of such fixed acids, that is, inorganic acids, non-volatile and non-decomposable by heat, such as silicic acid (silica, SiOz'), boric acid, stannic 'acid or compounds .or salts of these acids, that is, salts of those acids which have been defined as fixed acids and which show an acid reaction. These salts or fixed acids or fixed acid salts are salts which 'exert an action like an acid in that they have replaceable hydrogen therein. I

In some cases We have found it advisable to add, in addition to or inthe place of, these additional substances of acid reaction such compounds'as are capable of taking up alkali until they are fully saturated or neutralized. Sub-' stances of this kind are, for instance, metaborates,

for example, alkali metaborates, metasilicates, so-

called acid phosphates, such as NazHPOr and NaI-I2PO4, and the like. Through the addition of these substances the undesirable increase of substances of an alkaline nature in the melt, which have a disturbing effect on the cementation or.

times is not inconsiderable'.

carburization, orthe formation of such compounds may be prevented.

The best form in which these additional substances may be employed are their anhydrides, such as for instance, silica, boric acid anhydride and the like.

These substances are added advantageously in the same measure and the same proportion as the alkaline substances are formed in the melt. They may be added gradually and uniformly or in small doses, or also intermittently. They can also be added before the alkaline substances have been formed by the decomposition of the cyahides present, in order to prevent their formation altogether. The addition may be effected with advantage in connection with the restorationor the addition respectively of fresh carburizing agents such as cyanides, thereby replenishing the melt simultaneously and replacing any losses which have occurred by drag out or vaporization. The application of additionalsubstances according to our inventionnot only maintains the carburizing action of the bath, but also effects an increase in the carburizing action which some- This may be gathe'red from the following comparative tests, the

first of which serves as an example, too, for

carrying out the process of our invention:

' Example 1.-To a melt consisting of 13 kg. sodium chloride, 3.8 kg. potassium chloride and 1.6 kg. sodiumcyanide a quantity of 0.8 kg. silica. v in the shape of quartz sand is added. The

carburizin'g depth after two hours is 1.2 mm., after 4 hours 2.1 mm., after 6 hours 2.9 mm.,' one- "half to one-third of the carburizing layer being eutectic.

The working temperature during the immersion was 930.

Example 2.-When a melt of the same compositionis used without the addition of silica, and the working conditions are made absolutely equal, the cementation layer after 2 hours could not be discerned distinctly; It is 0.8 mm. after 4 hours, although it is not very distinct and under-eutectic. After 6 hours the thickness of the layer is about 1 mm. It is still not very distinct and undereutectic.

Example 3.To a melt containing 15 kg. barium chloride, 2.5 sodium chloride, 1.3 kg. potassium chloride and 2.5 kg. sodium cyanide is added after 6 hours a small amount of silica in pulverized form, and this addition is repeated as soon as the carburizing effect decreases noticeably. This may be, for instance, after further 8 hours, when the bath is replenished.

The process according to our inventionoffers advantages in a number of directions. It allows the prevention or the repression of the stabilizing action which is due to the alkaline substances formed during the cementation process by decomposition. It, further, permits of the regulation of the efliciency in the sense desired in each case, and thereby prevents the loss of expensive cementation agents. Furthermore, andjin particular, by the use of our process carburized layers can be obtained of such absolute thickness, such depths of glass-hard layers as have been unobtainable hitherto. In addition, no over-carburization occurs.

The transition of the layer is very, gradual and uniform and the melts are always fused salt bath containing alkali metal cyanide for carburlzing ferrous metals at temperatures below incipient fusion which consists in adding a substance selected from the group comprising silicic acid and silicic acid salts non-volatile and non-decomposable by heat at the temperature of the bath in such quantity and at such time as will increase, as desired, the efllcacy of the bath' for carburizing action.

5. Process for maintaining and regulating a fused salt bath containing alkali metal cyanide for carburizing ferrous metals at temperatures below incipient fusion which consists in adding a substance for neutralizing the alkalinity of the bath selected from the group comprising silicic acid, and silicic acid salts, and silicic acid anhydride, non-volatile and non-decomposableby heat'at the temperature of the bath, in such quantity and at such time as will increase, as desired, the emcacy' of the bath for carburizing action.

6. A fused salt bath containing alkali metal cyanide for carburizing ferrous metals at temperatures below incipient fusion containing substances for reducing the alkalinity as desired, selected from the group comprising silicic acid, silicic acid salts, and silicic acid anhydride, nonvolatile and non-decomposable by heat at the temperature of the bath.

WALTER BECK. KLAUS BONA'I'H.- 

